CYSTIC FIBROSIS TRANSMEMBRANE CONDUCTANCE REGULATOR

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This subproject is one of many research subprojects utilizing the resources provided by a Center grant funded by NIH/NCRR. The subproject and investigator (PI) may have received primary funding from another NIH source, and thus could be represented in other CRISP entries. The institution listed is for the Center, which is not necessarily the institution for the investigator. Cystic Fibrosis is (CF) is an autosomal recessive disorder caused by mutations in the Cystic Fibrosis Transmembrane Conductance Regulator (CFTR), the primary chloride (Cl-) channel in epithelial cells of numerous organ systems. In CF patients, a severe reduction in Cl- transport across epithelial cell membranes results in obstructive pulmonary disease, chronic sinusitis, pancreatic insufficiency, intestinal obstruction, and male infertility. While genotype-phenotype correlations have shown that some of the variablility in clinical presentation is due to specific mutations in CFTR, extreme variability exists among patients with identical mutations suggesting factors other than CFTR genotype may contribute to disease severity. Genes regulating normal CFTR function via upstream signaling events are rational candidate modifiers of CF. Upstream regulation of CFTR is cAMP dependent, and recent studies have shown CFTR mediated Cl- transport is modulated through phosphorylation by cyclic AMP (cAMP)-dependent protein kinase A. In epithelial cells cAMP levels are influenced by extra-cellular agonists via G-protein coupled transmembrane receptors. Thus, CFTR activity can be affected by G-protein coupled signaling processes that activate adenylyl cyclase and which generate cAMP. We are interested in investigating the role of genes in this upstream activation pathway in modulating the function of CFTR, and in turn disease severity. To this end, we propose to study CFTR function in patients with severe alterations in genes involved in this pathway. Patients with Albright's Hereditary Osteodystrophy (AHO) have been shown to have decreased cAMP levels in response to beta-adrenergic stimulation. Over the last decade this has been shown to be due to mutations in GNAS1, the gene that encodes the stimulatory alpha subunit of heterotrimeric G proteins (Gs-alpha). Patients with mutations in GNAS1 have a characteristic physical phenotype including short stature, brachydactyly, obesity, rounded facies, and subcutaneous ossifications. A subset of these patients also have hormone resistance, classically defined as decreased renal response to parathyroid hormone, but many patients have also been documented to have resistance to other hormones that signal via Gs-alpha. The exact mechanism for this hormone resistance is unclear, as patients within the same family and carrying the same mutation in GNAS1 can have variable phenotypes with regard to hormone resistance. We have preliminary data on one patient with AHO who has a decreased CFTR response to beta-agonists in the nasal epithelia and the skin. This suggests a link between altered Gs-alpha function and decreased CFTR function. To better understand the role of the cAMP-dependent pathway in modulating CFTR function, we propose to characterize CFTR function in patients with AHO. By examining GNAS1 RNA levels and cAMP levels in epithelial tissues, we will define the expected level of flux through the cAMP dependent pathway. CFTR function will then be assessed in these same tissues. The results of this study will define the role of the cAMP-dependent pathway in CFTR function, which may guide future therapeutic options for patients with CF. It may also address the mechanism of hormone resistance in AHO patients since it will be the first study directly characterizing, in vivo, the effects of GNAS1 mutation on one of its cellular targets. Results from family members will aid in genotype-phenotype correlations in AHO and clarify functional defects from familial variation.